In the semiconductor industry there is a continuing trend toward higher device densities. To achieve these high densities there have been, and continue to be, efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such a high device packing density, smaller feature sizes are required. These may include the width and spacing of interconnecting lines and the surface geometry such as the corners and edges of various features. The requirement of small features with close spacing between adjacent features requires high-resolution photo-lithographic processes as well as high resolution metrology and inspection instruments and systems.
Lithography refers generally to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which, for example, a silicon wafer is coated uniformly with a radiation-sensitive film (e.g., a photoresist), and an exposing source (such as ultraviolet light, x-rays, or an electron beam) illuminates selected areas of the film surface through an intervening master template (e.g., a mask or reticle) to generate a particular pattern. The exposed pattern on the photoresist film is then developed with a solvent called a developer which dissolves either the exposed pattern or the complimentary unexposed pattern, depending on the type of photoresist (i.e., positive or negative resist). After developing, the wafer has a photoresist mask corresponding to the desired pattern on the silicon wafer for further processing.
In addition to lithographic processes, other process steps in the fabrication of semiconductor wafers require higher resolution processing and measurement equipment in order to accommodate ever shrinking feature sizes and spacing. Measurement instruments and systems are used to inspect semiconductor devices in association with manufacturing production line quality control applications as well as with product research and development. The ability to measure and/or view particular features in a semiconductor workpiece allows for adjustment of manufacturing processes and design modifications in order to produce better products, reduce defects, etc. For instance, device measurements of film thicknesses, critical dimensions (CDs), profiles, and overlay registration may be used to make adjustments in one or more such process steps in order to achieve the desired product quality. Accordingly, various metrology and inspection tools and instruments have been developed to map and record semiconductor device features, such as scanning electron microscopes (SEMs), atomic force microscopes (AFMs), scatterometers, spectroscopic ellipsometers (SEs), and the like.
One particular type of measurement system is a scatterometer, which is different from conventional film measurements.. Scatterometry is a technique for extracting information about a structure or stacked structures upon which an incident light has been directed. In particular, scatterometry involves extracting information from gratings over other gratings or from gratings over a film stack. As indicated by its name, scatterometry is primarily concerned with the shapes of two and three dimensional structures in order to ascertain and determine the roughness of the layers or the non-planarity or non-parallelism of the planes. The structures of interest scatter light in ways that flat, one-dimensional layers do not. Process information concerning properties such as profile and critical dimensions of features present on and within the stacked structure can be extracted by employing a scatterometer. Using scatterometry, this information can be obtained by comparing measured and calculated signatures relating to the stacked structure. A signature may be defined as the phase and/or intensity of the light directed onto the surface of a wafer with phase and/or intensity signals of a complex reflected. and/or diffracted light resulting from the incident light reflecting from and/or diffracting through the surface upon which the incident light was directed.
Conventional film metrology involves treating volumes which are essentially one-dimensional, that is composed of layers such as sub-volumes separated by parallel planes. In scatterometry, the intensity and/or the phase of the reflected and/or diffracted light change according to properties of the stacked structure. Examples of such properties include the roughness of the layers and the non-planarity or non-parallelism of the subject plane(s) upon which the light is directed.
Different combinations of such properties will have different effects on the phase and/or intensity of the incident light resulting in substantially unique signatures in the complex reflected and/or diffracted light. Thus, by examining a database of calculated signatures or model of calculated signatures, a determination can be made concerning the properties of the stacked structure. For instance, a measured signature may be matched to a calculated signature, thereby yielding a measured profile of the stacked structure or a portion thereof. Such substantially unique signatures are produced by light reflected from and/or refracted by different surfaces due, at least in part, to the complex index of refraction of the surface onto which the light is directed.
The complex index of refraction (N) can be computed by examining the index of refraction (n) of the surface and an extinction coefficient (k). One such computation of the complex index of refraction can be described by the equation N=n−jk, where j is an imaginary number.
Generally, the n and k values for a given surface layer may be measured using a spectroscopic ellipsometer (SE), which may be used, in part, to generate such signature models in a semiconductor manufacturing endeavor. “Unpatterned wafer” means “an unpatterned portion of a wafer”. Optical instruments, e.g., an SE or a scatterometer, have a “spot size” of some size which defines the region where the instrument is sensitive. If a wafer has regions which are essentially uniform (“unpatterned”) as large as the spot size, those portions can be measured as “unpatterned” even though the wafer elsewhere has patterns, if the “spot” is placed on unpatterned on uniform regions. When exposed to a first incident light of known intensity, wavelength and phase, a first layer with a first chemical composition on a wafer can generate a first phase/intensity signature. Similarly, when exposed to the first incident light of known intensity, wavelength and phase, a second chemical composition on a wafer can generate a second phase/intensity signature. For example, a nitrided gate oxide layer with a first nitrogen concentration may generate a first signature while a nitrided gate oxide layer with a second nitrogen concentration may generate a second signature.
Observed signatures can be combined with simulated and modeled signatures to form the signal (signature) library. Simulation and modeling can be employed to produce signatures against which measured signatures can be matched, for instance, using a profile matching server or system. When phase/intensity signals are received from scatterometry detecting components, the phase/intensity signals can be pattern matched, for example, to the library models of signals, in order to determine whether the signals correspond to a stored signature.
Scatterometry may thus be advantageously employed in a semiconductor device manufacturing or fabrication process, in order to measure certain process parameters associated with individual processing steps therein. For example, a lithography process step may involve patterning wafers in order to create features thereon having certain critical dimensions (CDs), profiles, spacings, etc., wherein the overall quality of the resulting semiconductor device may depend on the accuracy of the lithography step. Scatterometry may be employed in order to verify such dimensional process parameters, as well as other process conditions, such as overlay registration, and the like. Today, such model generation is typically done remotely from the process and scatterometer with which the models are ultimately to be employed. In order to setup a scatterometer for use with a new or changed process step, such models must be obtained, along with recipes for performing one or more required measurements on processed wafers.
Obtaining models from such remote model generation sites sometimes takes days, during which time wafers processed according to the new process step cannot be measured using the scatterometer. In addition, the scatterometry measurement system may need to be trained in order to program new measurement recipes, during which time the scatterometer cannot be used to measure production wafers. Thus, the generation and/or creation of setup information such as models and recipes for use in measurement systems has heretofore resulted in significant process down-time. Accordingly, there is a need for improved methods and systems by which such setup down-time may be reduced or mitigated.